Microchannel devices are finding increased use in the identification and synthesis of chemical and biological species. Employing transverse channel dimensions from a few microns to about one millimeter, such systems may permit the miniaturization and large-scale integration of many chemical processes in a manner analogous to that already achieved in microelectronics. Applications for microchannel devices now under development include such diverse processes as DNA sequencing, immunoassay, the identification of explosives, identification of chemical and biological warfare agents, and the synthesis of chemicals and drugs.
Most microchannel systems for chemical and biological analysis employ some variant of electrochromatographic or electrophoretic separation. In chromatographic processes, bulk electroosmotic motion of a fluid is induced by applying an electric field along the length of the separation column. Individual species move through the column at various speeds due to preferential adsorption on stationary surfaces such as the channel walls or an internal porous packing. In contrast, electrophoretic processes involve little or no bulk fluid motion. Here the applied electric field instead produces motion of ionic species through a stationary or nearly stationary transport medium that may be either a fluid or a gel. Species separation in electrophoretic processes occurs as a result of differing ratios of the ion charge to the ion mobility and consequent differing ion speeds.
Electroosmotic flows offer two important benefits over pressure-driven flows for transport processes in microchannel devices. First, transport speeds in electroosmotic flow are independent of the width and depth of a channel cross-section over a wide range of conditions, making this technique for driving fluid motion extensible to extremely small physical scales. In contrast, pressure-driven flows require a pressure gradient that increases inversely with the square of the minimum transverse dimension to maintain a given fluid speed. Second, the profile of the fluid velocity across the cross-section of a long straight channel is essentially flat in electroosmotic flows, again over a wide range of conditions. All transverse variation in the axial speed is confined to a small region adjacent to the channel walls and comparable in thickness to the electric Debye layer. The benefit of this flat velocity profile is that samples may be transported over long ranges with very little hydrodynamic dispersion due to nonuniform fluid speeds.
Electrophoretic processes offer somewhat different benefits in microchannel devices. Because the overall dimensions of microchannel devices tend to be only a few centimeters, very large electric fields may be produced by relatively small electric potentials. This permits larger ion speeds and reduces the overall time for separation processes. Electrophoretic motion is also easy to produce in microchannel systems since, like electroosmotic flow, only electrodes and a power supply are required to produce the phenomenon. Mechanical pumps are unnecessary. Also like electroosmotic flows, this method of producing species motion introduces little extraneous spreading of a sample band as it moves along the separation column since the electric field in a long straight channel is spatially uniform.
Despite these benefits in long-range transport, electrokinetic mechanisms are not particularly well suited to producing a thin sample band for subsequent processing in a separation or other process channel. The reason for this is that species motion in both electroosmotic and electrophoretic transport processes is governed by the highly-diffusive Laplace equation, and the region over which motion is induced usually occupies at least one channel width. As a result, the thickness of a sample band produced by such motion will usually exceed the channel width; this is not acceptable for many processes of practical interest.
For electrochromatographic and electrophoretic separation processes, the thickness of the initial sample band measured in the direction of the sample motion may need to be small compared to the channel width. Separation columns in microchannel systems are typically not very long, so final spacings between constituent bands may span only a channel width or so. To resolve these bands requires that the thickness of the sample band initially injected into the column is much smaller than the band spacing or, equivalently, much smaller than the channel width. In addition to separation processes, small sample sizes and sharp definition of species interfaces are also desirable during routine sample transport. These desired characteristics allow more precise control over processes such as mixing, dilution and synthesis.
The invention described here provides a method and apparatus for producing a sample band for subsequent processing wherein the thickness of the band in the direction of sample motion is small compared to the channel width. Further, the thickness of the band may be controlled to provide a desired sample volume or size. This method can even produce sample bands having an overall thickness an order of magnitude or so smaller than the channel width. The method can be implemented using conventional channel geometries and conventional electrical hardware. Additional benefits can be obtained by using these new methods in conjunction with improved channel geometries. Such improved geometries are also disclosed here.